K5 left Torbay on 19 January 1921 with the K8, K10, K15 and K22 as part of the Atlantic Fleet for a mock battle in the Bay of Biscay.

[1] An oil slick was discovered and after planks from the battery covers and a sailor's "ditty box" were recovered, it was presumed that she had somehow gone past her maximum depth and been crushed.

On return from her exercises in the Mediterranean in 1922, Hood and the rest of the fleet dropped wreaths and held a memorial service where K5 had gone down.

Retired Rear-Admiral S.S. Hall wrote in The Times under the heading "An Experts Theory" [2] "...it may be taken as certain that the loss of the vessel was due to some delay to checking the downward momentum gained by the vessel being overtrimmed in diving, either by admitting compressed air too slowly to too many tanks at one time, to tanks only partially full, or to a sea connexion being closed prematurely."

"The high surface speed necessitates great length, and the further complication of steam demands very large openings for funnels and air intakes to boiler rooms.